The Sangheili-San 'Shyuum War was an interstellar war set between the Sangheili and the San 'Shyuum races, over religious and technological differences. At the conclusion of the war, the Writ of Union was penned and initiated, and the Covenant was kindled.
After leaving their homeworld in 2100 BCE, the San 'Shyuum encountered the Sangheili in 938 BCE, who also had experience with Forerunner technology. The San 'Shyuum believed that incorporating Forerunner technology was allowed by their religion. To the Sangheili, such a thing would have been unthinkable heresy.
During the combat, the Sangheili had an early advantage, with each warrior comparable in skill to several San 'Shyuum in battle. However, the San 'Shyuum's technological superiority, and possession of the Forerunner Dreadnought, meant that they had the advantage in space battles, using the Dreadnought to great effect in devastating hit-and-run strikes against the Sangheilian War Fleets. Eventually in 876 BCE, in order to ensure the survival of their race, the Sangheili were forced to amend their position of the treatment of Forerunner technologies and began studying their relics and incorporating elements into their warship, weapon and armor designs. The San 'Shyuum also realized that if there were other races with martial capabilities like the Sangheili, they would be in serious danger.
In time, the Sangheili learned of the San 'Shyuum's belief in the Great Journey, where the Forerunners used the Halo Array to ascend to a god-like state, and wished to repeat this process on themselves. In 852 BCE, the two races signed a treaty known as the Writ of Union to form the early Covenant, which defined the two races positions: the San 'Shyuum would act as the head religious leaders, studying the Forerunners' technology and seek out the Halos, while the Sangheili would serve as their protectors, using weapons and technology developed by the San 'Shyuum. The Dreadnought was decommissioned and made the center point and power source of High Charity, a mobile planetoid which would serve as the capital of the Covenant.